Directional solidification apparatus and related methods

ABSTRACT

A directional solidification apparatus includes a mold heating chamber, a solidification chamber, and a gas source. The solidification chamber is adjacent the mold heating chamber for solidifying molten metal formed from an air melt allow system as a cast body as the metal is withdrawn from the mold heating chamber. The gas source is in fluid communication with the mold heating chamber for providing a pressurized atmosphere for directionally solidifying metal as a cast body having single crystal or multi-crystal columnar micro structure.

RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 61/921,574 filed Dec. 30, 2013, the contents ofwhich are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to apparatus and methods for casting, andmore particularly to apparatus and methods for directionally solidifyingcast bodies.

2. Description of Related Art

Gas turbine engines include components that can be subject to extremetemperature and stress during engine operation. Such components, likeblades, vanes, and blade outer air seals, are typically constructed fromnickel-based superalloy castings because the high number of slip planespresent in the face-centered cubic microstructures of such materials iswell suited to extreme temperature and high stress applications.Examples of castings formed from nickel-based alloys and superalloys aredescribed in U.S. Pat. No. 3,260,505 to Ver Synder and U.S. Pat. No.3,494,709 to Piearcy, the contents of which are incorporated herein byreference in their entirety.

Nickel-based alloy and superalloy castings, i.e. vacuum melt alloysystems, are generally formed by directionally solidifying molten metalin dual chamber vacuum induction furnaces.

Such furnaces typically include an upper mold heating chamber forreceiving molten metal in a mold and a lower cooling chamber is adaptedfor maintaining a steep thermal gradient within the mold as the mold iswithdrawn from the upper chamber into the lower chamber. Solidificationgenerally occurs under vacuum, thereby developing a nickel-based alloyor superalloy casting with single crystal or multi-crystal columnarmicrostructure. Examples of induction furnaces suitable for vacuum meltalloy systems include those described in U.S. Pat. No. 3,763,926 toTschinkel, and U.S. Pat. No. 4,108,236 to Salkeld, the contents of whichare herein incorporated by reference.

Such conventional apparatus and methods have generally been consideredsatisfactory for their intended purpose. However, there is a need in theart for induction furnaces and casting methods suitable for castingsingle crystal or multi-crystal columnar castings formed from air meltalloy systems, such as carbon steel or low-alloy steels for example. Thepresent disclosure provides a solution for this need.

SUMMARY OF THE INVENTION

A directional solidification apparatus includes a mold heating chamber,a solidification chamber, and a gas source. The solidification chamberis adjacent the mold heating chamber for directionally solidifying acast body as the body is withdrawn from the mold heating chamber. Thegas source is in fluid communication with the mold heating chamber forproviding a pressurized atmosphere to the molten metal duringsolidification for solidifying the cast body as a single crystal ormulti-crystal columnar cast body.

In certain embodiments, the mold is configured for directionallysolidifying a charge of molten metal formed from an air melt alloysystem such as carbon steel, low alloy steel, or non-nickel based alloyunder an inert or oxidizing environment. A valve, such as a gate valve,can be operatively associated with apparatus for selectively placing theinterior of the apparatus in fluid communication with the gas source. Acooling module can provide cooling to the valve.

In accordance with certain embodiments, the gas source can be anoxidizing gas source or an inert gas source, such as argon, nitrogen, ormixtures thereof. A heating element can be arranged within an interiorportion of the mold heating chamber. A baffle can separate the moldheating chamber from the solidification chamber for limiting radiantheating of the solidification chamber. The baffle can be constructedfrom an oxide-based ceramic material or a material suitable for use in ahigh-temperature environment with an oxidizing or inert atmosphere, suchas alumina, partially stabilized zirconia, alumina-silicate, orcordierite for example.

It is contemplated that the apparatus include a gas impingement modulein fluid communication with the solidification chamber for removing heatfrom the directionally solidified cast body using air. A water ring canbe disposed within the solidification chamber for removing heat from thecast body using a liquid cooling medium. The interior of the apparatuscan be a hyperbaric controlled environment for reducing volatilemigration from the metal as it solidifies into a cast body. The interiorof the apparatus can provide a controlled, low vacuum environment fordirectionally solidifying the cast body with single crystal ormulti-crystal columnar microstructure.

A method of casting air melt alloy systems includes introducing moltenmetal formed from an air melt alloy system into a mold heating chamberunder a controlled atmosphere, withdrawing the molten metal from themold heating chamber and into a solidification chamber adjacent the moldheating chamber under the controlled atmosphere, and removing heat fromthe molten metal, thereby forming a directionally solidified cast bodyformed from a single crystal or a multi-crystal columnar microstructurewithin the controlled atmosphere.

These and other features of the systems and methods of the subjectdisclosure will become more readily apparent to those skilled in the artfrom the following detailed description of the preferred embodimentstaken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

So that those skilled in the art to which the subject disclosureappertains will readily understand how to make and use the devices andmethods of the subject disclosure without undue experimentation,preferred embodiments thereof will be described in detail herein belowwith reference to certain figures, wherein:

FIG. 1 is a cross-sectional side view of a casting apparatus constructedin accordance with the present disclosure, showing an apparatus interiorfor solidifying molten metal within an inert atmosphere;

FIG. 2 is a cross-sectional side view of a second embodiment of acasting apparatus constructed in accordance with the present disclosure,showing an apparatus interior for solidifying molten metal within anoxidizing atmosphere;

FIG. 3 is a cross-sectional side view of a third embodiment of a castingapparatus constructed in accordance with the present disclosure, showingan apparatus for solidifying molten metal within an inert or oxidizingatmosphere using a liquid metal bath;

FIG. 4A-FIG. 4D are cross-sectional views of a directionally solidifiedcast body in accordance with the present disclosure after etching with afirst reagent, showing body microstructure; and

FIGS. 5A-5D are cross-sectional views of the directionally solidifiedcast body after etching with a second reagent, showing bodymicrostructure; and

FIG. 6 is a method of directionally solidifying molten metal comprisedof an air melt alloy system as a cast body having single crystal ormulti-crystal columnar microstructure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to the drawings wherein like referencenumerals identify similar structural features or aspects of the subjectdisclosure. For purposes of explanation and illustration, and notlimitation, a partial view of an exemplary embodiment of a castingapparatus in accordance with the disclosure is shown in FIG. 1 and isdesignated generally by reference character 100. Other embodiments ofthe casting apparatus in accordance with the present disclosure, oraspects thereof, are provided in FIGS. 2-6, as will be described. Thesystems and methods described herein can be used for directionallysolidifying molten metal comprising air melt alloy systems as castingshaving single crystal or multi-crystal columnar microstructure.

With reference to FIG. 1, a casting apparatus 100 is shown. Castingapparatus 100 includes a mold heating chamber 110, a solidificationchamber 120, a gas source 130, and a baffle 140. Casting apparatus 100is operatively associated with a melt box 150 and a withdrawal mechanism160. Casting apparatus 100 includes a mold 170 movably disposed withinits interior for receiving a charge of molten metal. It is contemplatedthat the molten metal comprises an air melt alloy system, such as carbonsteel, low alloy steel or copper-nickel alloy for example.

Mold heating chamber 110 is arranged above and adjacent tosolidification chamber 120. Baffle 140 separates mold heating chamber110 from solidification chamber 120 and has an aperture configured toconform to a portion of mold 170 disposed within the aperture. Melt box150 is operatively associated with mold heating chamber 110 and isconfigured for transferring molten metal into mold 170 when mold 170 ispositioned in an upper portion of mold heating chamber 110. Withdrawalmechanism 160 is operatively associated with mold heating chamber 110and solidification chamber 120 and configured for transferring mold 170from mold heating chamber 110 into solidification chamber 120 alongwithdrawal axis W.

Mold heating chamber 110 has an interior 111 configured for beingpneumatically isolated from the atmosphere external to apparatus 100.Mold heating chamber 110 includes an insulating body 112, heatingelements 114 such as induction coils or resistive heating elements, avalve 116, and a susceptor 118. Heating elements 114 are disposed withinmold heating chamber 110 between insulating body 112 and susceptor 118and are in thermal communication with susceptor 118. Susceptor 118 is agraphite body configured for uniformly distributing heat generated byheating elements 114 within interior 111 of mold heating chamber 110.Insulating body 112 also has an aperture disposed in its upper portionconfigured for receiving molten metal from melt box 150 and selectivelyseparating interior 111 of mold heating chamber 110 from the atmosphereexternal to apparatus 100. Baffle 140 bounds mold heating chamber 110 onits lower portion and separates interior 111 from solidification chamber120, thereby reducing radiant heating of solidification chamber 120 byelements within mold heating chamber 110.

Solidification chamber 120 includes a housing 121 defining an interior122 and an isolation valve 126. Interior 122 is configured to receivemold 170 as mold 170 advances along withdrawal axis W. Interior 122 isbounded on its upper end by baffle 140 and by housing 121 about itsperiphery. Housing 121 optionally includes a water cooled chill ring190. Water cooling ring 190 can be in fluid communication with a supplyof liquid coolant, e.g., water, and in thermal communication withinterior 122. Isolation valve 126 is configured to separation moldheating chamber 110 from solidification chamber 126 once mold 170 hasbeen withdrawn below isolation valve 126. This allows for removing mold170 without exposure of the interior of mold heating chamber 110 to theatmosphere external to apparatus 100.

Gas source 130 includes a gas source 132, a vacuum source 134, and valve116. Gas source 132 is in selective fluid communication with interior111 through valve 116. Vacuum source 134 is also in selective fluidcommunication with interior 111 through valve 116. Valve 116 isconfigured for selectively placing gas source 130 and vacuum source 134in selective fluid communication through valve 116 with interior 111,thereby controlling the internal atmosphere of apparatus 100 duringsolidification of molten metal disposed within mold 170. This allows forevacuating interior 111 of air and charging interior 111 with an inertatmosphere. Charging interior 111 with an inert atmosphere in turnprevents evaporation of air melt alloy constituents with relatively lowvapor pressures, such as chromium or aluminum, potentially changing theconstitution of the alloy forming cast body 10 (shown in FIG. 4 and FIG.5) from that of the molten metal delivered to mold 170. This can alsoreduce or prevent defects from forming in the body during solidificationthat could otherwise develop due to the relatively low vapor pressure ofconstituents of the molten metal were the body solidified under vacuum.

It is contemplated that the gas source can be an inert gas source. Thegas source can be a nitrogen supply or an argon supply for directionallysolidifying cast body 10 (shown in FIG. 4 and FIG. 5) in a nitrogenatmosphere or an argon atmosphere. It is also contemplated that vacuumsource 134 can be configured to evacuate interiors 111 and/or 122 andbackfill interiors 111 and 122 with an inert gas at a controlledpressure. The controlled pressure can be hyperbaric, e.g. above 1atmosphere. Alternatively, the controlled pressure can be hypobaric,e.g. between about 0.5 atmosphere to about 1 atmosphere (0.506 bar toabout 1.013 bar).

Valve 116 can be a gate valve. Valve 116 can optionally be provisionedwith cooling such that heat conducted to valve 116 by the atmospherewithin apparatus 100 does not adversely impact the reliability of valve116.

Melt box 150 includes a vessel 152 and heater elements 154 operativelyassociated with vessel 152. Heater elements 154 are configured forheating vessel 152 and contents thereof. This enables delivering acharge of molten metal comprising an air melt alloy system to mold 170.Heater elements 154 can be induction coils or resistive heatingelements, for example.

Withdrawal mechanism 160 is operatively associated with mold 170 andincludes a chill plate 174. Withdrawal mechanism 160 connects to a lowerportion of mold 170 and is configured for displacing mold 170 betweeninterior 111 of mold heating chamber 110 and interior 122 ofsolidification chamber 120 along withdrawal axis W. Withdrawal mechanism160 is also configured for positioning mold 170 in an upper portion ofinterior 111 to receive molten alloy from melt box 150. Withdrawalmechanism 160 thereafter progressively withdraws mold 170 throughinterior 111 and into interior 122, maintaining a consistent thermalgradient within mold 170 for directionally solidifying molten metaldisposed within mold 170 as a cast body 10 (shown in FIG. 4 and FIG. 5)with single crystal or multi-crystal columnar microstructure developedunder an inert atmosphere.

Mold 170 can be a ceramic shell mold with cavities 172 for forming castbodies 10 (shown in FIG. 4 and FIG. 5). Cavities 172 have axiallyextending shape with a lower portion configured for transferring heatfrom the lower portions of cavities 172. It is contemplated thatcavities 172 can have the inverse shape needed for forming a componentthat can exploit the advantages of a single crystal structure, e.g.creep resistance, such as turbine blades for example. Cavities 172 areconfigured for receiving molten metal and transferring heat from themolten metal through a chill plate coupled to a lower portion of mold170.

With reference to FIG. 2, a casting apparatus 200 is shown. Castingapparatus 200 is similar to casting apparatus 100 and is additionallyconfigured for directionally solidifying cast bodies 10 (shown in FIG. 4and FIG. 5) in an oxidizing environment. Casting apparatus 200 includesa mold heating chamber 210, a solidification chamber 220, a gas source230, and a baffle 240. Casting apparatus 200 also includes an airimpingement module 280 and a water cooling ring 290.

Gas source 230 is configured for providing and sustaining an oxidizingatmosphere within either or both of an interior 211 of mold heatingchamber 210 and an interior 222 of solidification chamber 220 forsolidifying molten metal introduced into mold 170. Mold heating chamber210 and baffle 240 are constructed on an inflammable material forwithstanding the high temperature oxidizing environment maintainedwithin interior 211 while directionally solidifying molten metal withinmold 170.

Gas impingement module 280 is in fluid communication with interior 222of solidification chamber 220, and provides a flow of compressed air,nitrogen, helium, argon, or other suitable compressed medium to mold 170for cooling molten metal disposed therein. Water cooling ring 290 is influid communication with a supply of liquid coolant, e.g., water, and isin thermal communication with interior 222. Each gas impingement module280 and water cooling ring 290 are configured for removing heat from themolten alloy within mold 170 as it advances along withdrawal axis W,thereby maintaining a suitable thermal gradient within mold 170 fordeveloping cast bodies 10 (shown in FIG. 4 and FIG. 5) having singlecrystal or multi-crystal columnar microstructure.

Conventional susceptor and baffle assemblies used for vacuum melt alloysystems are generally constructed from materials unsuitable foroxidizing environments, such as graphite. Because apparatus 200directionally solidifies molten metal within an oxidizing atmosphere,apparatus 200 includes baffle 240 constructed from material suitable foruse in a high-temperature environment with an oxidizing atmosphere.Examples of such materials include oxide-based ceramic materials,alumina, partially stabilized zirconia, alumina-silicate, or cordierite.Baffle 240 can also be constructed from compressed fibers, such asaluminosilicate-based fiber board for example. This potentially providesa casting environment suitable for developing cast bodies 10 (shown inFIG. 4 and FIG. 5) formed from air melt alloy system with single crystalor multi-crystal columnar microstructure. It is also contemplated thatbaffle 240 can be constructed from individual leaves configured formoving as the mold advances into the solidification chamber, therebyconforming to variation in the cross-sectional shape of mold 170. Baffle240 can also be a static structure configured to remain fixed as themold advances into the solidification chamber.

Notably, casting apparatus 200 does not include a susceptor constructedfrom graphite. Instead, casting apparatus 200 includes heating elements214 distributed within interior 211 to achieve similar heating effect asthat achieved using a susceptor. This allows for directionallysolidifying air melt allow systems as cast bodies with single crystal ormulti-crystal columnar microstructure and preventing evaporation ofalloy constituents with low vapor pressure into the chamber atmosphere,such as chromium or aluminum, potentially changing the constitution ofthe alloy forming cast body 10 (shown in FIG. 4 and FIG. 5) from that ofthe molten alloy delivered to mold 170. Moreover, it also allows fordirectionally solidifying cast bodies within apparatus 200 within anoxidizing atmosphere such as air that is readily available andrelatively inexpensive.

With reference to FIG. 3, a casting apparatus 300 is shown. Castingapparatus 300 is similar to casting apparatus 100 and additionallyincludes a liquid metal bath 322, skimming mechanism 326, and baffle340. Withdrawal mechanism 160 is operatively associated with mold 170for driving mold 170 into liquid metal bath 322. Liquid metal bath 322is adapted for receiving mold 170 as it advances along withdrawal axis Wand transferring heat therefrom, thereby assisting water-cooled chillplate 174 in maintaining a thermal gradient within mold 170. Liquidmetal bath 324 can include tin, indium, copper, copper-indium,copper-antimony, aluminum, or aluminum-copper by way of non-limitingexample.

Baffle 340 is configured for maintaining heat within liquid metal bath324 and is optional in embodiments of casting apparatus 300 configuredfor certain liquid metal cooling processes. In embodiments includingbaffle 340, baffle 340 prevents liquid metal from evaporating fromliquid metal bath 324 into the atmosphere of interior 311 of moldheating chamber 310. This allows for maintaining relatively low vaporpressures within mold heating chamber interior 311 of mold heatingchamber 310, e.g. less than 1 atmosphere (about 101 kilopascals).

With reference to FIGS. 4A-4D, views of a first transverse section 12 ofan example cast body 10 is shown. Example cast body 10 is a singlecrystal cast steel body formed from carbon steel alloy conforming tocurrent AMS5362 specifications, e.g. AMS5362 rev 9, formed using castingapparatus 100. First transverse section 12 is a cross-section taken inan x-y plane orthogonal with respect solidification axis z(corresponding to withdrawal axis W discussed above). Prior to acquiringthe images presented in FIG. 4, transverse section 12 was etched usingFry's Reagent to expose dendrites 14 and grain boundaries as applicable.This was accomplished by immersing a transverse section of example castbody 10 taken orthogonally with respect to the crystal growth axis andswabbing the section surface with a mixture of about 5 grams of copperchloride per 40 milliliters of hydrochloric acid, 25 milliliters ofethanol, and 30 milliliters of water.

FIG. 4A and FIG. 4C show microstructure of first transverse section 12magnified 50 times. FIG. 4B shows microstructure of first transversesection 12 magnified 75 times. FIG. 4D shows microstructure of firsttransverse section 12 magnified 400 times.

Notably, no grain boundaries are visible in the transverse sectionalimages shown in FIGS. 4A-4D. This indicates that the AMS5362 materialforming example cast body 10 has a single crystal microstructure. Alsonotable in FIGS. 4A-4D are that the dendrites formed within themicrostructure have primary and secondary orientations that aresubstantially orthogonal with respect to one another. This indicatesthat cast bodies formed from air melt alloy systems such as AMS5362(shown) are amenable to seeding for controlling both the primary andsecondary solidification orientations of the material.

With reference to FIGS. 5A-5D, a second transverse section 14 of examplecast body 10 is shown. Second transverse section 14 is similar totransverse section 12 with the difference that the section was etchedusing Kialing's Reagent. Kialing's reagent is a mixture of aboutcontaining 5 grams of copper chloride per 100 milliliters ofhydrochloric acid and 100 milliliters of ethanol. The reagent wasapplied to second transverse section 14 for purposes of making themicrostructure of example cast body 10 readily visible for opticalinspection.

FIG. 5A shows microstructure of second transverse section 14 magnified38 times. FIG. 5B shows microstructure of second transverse section 14magnified 74 times. FIG. 5C shows microstructure of second transversesection 14 magnified 150 times. FIG. 5D shows microstructure of secondtransverse section 14 magnified 350 times.

Notably, no grain boundaries are visible in FIGS. 5A-5D. The lack ofgrain boundaries indicates that directionally solidified example castbody 10 has single crystal microstructure. Dendrites visible in FIGS.5A-5D show primary and secondary orientations orthogonal with respect toone another, indicating once again that the cast carbon and low alloysteels such as AMS5362 are amenable to seeding processes used fornickel-based superalloys for controlling crystal growth.

While a single example material is illustrated in the accompanyingfigures, it will be appreciated that the apparatus and methods describedherein are suitable for other air melt alloy systems such as stainlesssteels, monel (i.e. copper-nickel alloy systems), brass,copper-chromium, or high-alloy coppers such as GRCop 84 conforming withNASA/TM-2005-213566 specifications. While single crystal microstructureis illustrated in the accompanying figures, it will also be appreciatedthat cast bodies formed using directional solidification such as throughdirectionally solidified columnar casting can also be formed using theapparatus and methods described herein.

With reference to FIG. 6, a method 400 of forming a cast body is shown.Method 400 includes the steps of (a) introducing 410 molten metalcomprised of an air melt alloy into a mold heating chamber in acontrolled atmosphere, (b) withdrawing 420 the molten metal into asolidification chamber in the controlled atmosphere, and (c) removing430 heat from the molten metal under positive pressure to form a singlecrystal or multi-crystal columnar cast body formed from the air meltalloy system in the controlled atmosphere. The controlled atmosphere canbe a positive pressure atmosphere, such as an inert atmosphere oroxidizing atmosphere as described above.

Controlling the atmosphere within which molten air melt alloys such ascarbon steel or low alloy steel is solidified can reduce splittingand/or alloy volatiles from exiting the molten material duringsolidification. This allows for forming cast bodies formed from air meltalloy systems with single crystal or multi-crystal columnarmicrostructure without significant alterations of the alloy chemistrythat could otherwise develop during solidification of the due to thevapor pressure(s) of some alloying constituents present in the alloy.Such cast bodies in turn can have superior mechanical properties, suchas creep resistance, thereby allowing for construction of gas turbineengine components such as turbine blade which are currently limited tonickel-based steels and/or superalloys.

The methods and systems of the present disclosure, as described aboveand shown in the drawings, provide for casting apparatuses andtechniques with superior properties including the ability todirectionally solidify castings as a single crystal or columnar castingsformed from non-esoteric (or exotic) air melt alloy systems. This canprovide materials with anisotropic physical properties suitable forapplications presently served by materials with isotropic properties butwhich could benefit from materials with anisotropic properties byadapting design methodologies known in aerospace but not generallyapplied in other applications, such as automotive and other industrialapplications for example. While the apparatus and methods of the subjectdisclosure have been shown and described with reference to preferredembodiments, those skilled in the art will readily appreciate thatchanges and/or modifications may be made thereto without departing fromthe spirit and scope of the subject disclosure.

What is claimed is:
 1. A directional solidification apparatus,comprising: a mold heating chamber; a solidification chamber adjacentthe mold heating chamber for solidifying molten metal comprising an airmelt alloy system as the metal is withdrawn from the mold heatingchamber; and a gas source in fluid communication with the mold heatingchamber, wherein the gas source is configured to provide a pressurizedatmosphere for directionally solidifying the metal as a cast body withsingle crystal or multi-crystal columnar microstructure.
 2. An apparatusas recited in claim 1, wherein apparatus is configured for solidifying acast body formed from carbon steel, low alloy steel, or a non-nickelbased alloy system.
 3. An apparatus as recited in claim 1, wherein thegas source an inert gas source.
 4. An apparatus as recited in claim 1,wherein the inert gas source is an argon gas source.
 5. An apparatus asrecited in claim 1, wherein the gas source is an oxidizing gas source.6. An apparatus as recited in claim 1, further including a valve fluidlycoupling the gas source with the solidification chamber for providing aninert solidification atmosphere during solidification of the cast body.7. An apparatus as recited in claim 1, further including a heatingelement arranged within an interior chamber of the mold heating chamber.8. An apparatus as recited in claim 1, further including a baffleseparating the mold heating chamber from the solidification chamberconstructed from a material suitable for use in a high-temperatureenvironment with an oxidizing atmosphere.
 9. An apparatus as recited inclaim 1, wherein the solidification chamber includes a liquid metal bathfor removing heat from the metal.
 10. An apparatus as recited in claim8, wherein the baffle is constructed from a material selected from thegroup consisting of alumina, partially stabilized zirconia,alumina-silicate, cordierite, and an oxide-based ceramic.
 11. Anapparatus as recited in claim 1, further including an air impingementmodule in fluid communication with the solidification chamber forremoving heat from the cast body using air.
 12. An apparatus as recitedin claim 1, further including a water ring disposed within thesolidification chamber for removing heat from the cast body using aliquid coolant.
 13. An apparatus as recited in claim 1, wherein the gassource is configured for evacuating an interior of the apparatus andcharging the atmosphere with an inert gas for solidifying the cast body.14. An apparatus as recited in claim 1, wherein an interior of theapparatus is a hyperbaric controlled environment for reducing migrationof volatile alloy constituents from molten metal to the apparatusinterior during solidification of the cast body.
 15. An apparatus asrecited in claim 1, wherein an interior of the apparatus is a hypobariccontrolled environment for reducing migration of volatile alloyconstituents from molten metal to the apparatus interior duringsolidification of the cast body.
 16. A directional solidificationapparatus, comprising: a mold heating chamber; a solidification chamberadjacent the mold heating chamber for solidifying molten metalcomprising an air melt alloy system as the metal is withdrawn from themold heating chamber; a baffle separating the mold heating chamber fromthe solidification chamber; and a gas source in fluid communication withan interior of the mold heating chamber through a cooled valve, whereinthe gas source is configured for charging apparatus interior with aninert atmosphere for directionally solidifying the metal as a cast bodyhaving single crystal or a multi-crystal columnar microstructure.
 17. Amethod of casting steel, comprising: introducing molten metal comprisingan air melt alloy system into a mold heating chamber in a controlledatmosphere; withdrawing the molten metal into a solidification chamberin the controlled atmosphere; and removing heat from the molten metal ina controlled atmosphere and developing a directionally solidified castbody with single crystal or a multi-crystal columnar microstructure. 18.A method as recited in claim 17, wherein the controlled atmosphere is aninert atmosphere.
 19. A method as recited in claim 17, wherein thecontrolled atmosphere is a positive pressure oxidizing atmosphere.
 20. Amethod as recited in claim 17, wherein the controlled atmosphere is alow vacuum oxidizing atmosphere.